Water Content Determination of Soil

The water content of a soil is very important parameter as it influences the soil behaviour (especially that of cohesive soils). For any sample of soil, the very test done is water content determination. This is because of mainly two reasons

1. Since during transportation and storage water content may change. Hence it is important to determine the water content before carrying out other tests of soil.
2. Some of the physical properties of practical importance are calculated using water content for e.g. dry unit weight, bulk unit weight etc.

Water content is a quantitative measure of the wetness of the soil. The water content can be determined to a high degree of precision as it involves mass only which can be determined with fair accuracy than volumes.

The water content of soil can be determined from any one of the following methods:

Oven Drying Method

In this method, the soil is taken in a small, non-corrodible, airtight container and then the soil sample in the container is dried at a temperature of 105-110°C for almost 24 hours (if the temperature is increased above 110°C then the water of crystallization may be lost). Also for soils containing more organic matter, the temperature is maintained between 60°C to 80°C since organic content in soil may decompose and get oxidized. Measurement of weight has been taken before and after drying and then water content is determined using the formula:

Torsion Balance Method

This method is implemented for the water content determination of the soils which quickly re-absorb moisture after drying. In this method, drying and weighing occur simultaneously. The infrared lamp and torsion balance meter are used for rapid and accurate determination of the water content. The equipment has two main parts viz. the infrared lamp and torsion balance.

Pycnometer Method

A pycnometer is a jar of capacity 1 litre and made of glass with a brass conical cap. The cap has a hole of 6 mm diameter at its apex. This method can be used to determine water content if the specific gravity of soil is known. In this method, four weight measurement are taken which are as follows:

1. The weight of empty pycnometer
2. The weight of pycnometer + moist soil (placed inside the pycnometer)

After taking the weight of pycnometer with moist soil (generally one-third of the volume) inside it, the remaining volume of pycnometer is filled with water.

3. The weight of pycnometer + soil + water

Then the pycnometer is emptied and filled completely with water only, so the fourth measurement is:

4. The weight of pycnometer + Water

In this method, instead of water, kerosene can also be used.

Sand Bath Method

This method is used in the field to determine the water content of the soil. The method is not very much accurate however the method is quick for the water content determination. A sand bath is actually a large, open vessel containing sand filled to a depth of 3 cm or more. The soil sample is crumbled and loosely placed in a tray. A few pieces of white paper are placed above the sample. The weight of the tray with the sample is measured and denoted as the mass of the wet sample.

The tray is placed on the sand bath and sand bath is heated over a stove. it takes around 20 to 60 minutes for the soil sample to dry. The soil should not be overheated. The turning of white paper to brown indicates the overheating of the sample. Drying should be continued till the sample attain the constant mass. After completion of drying, the weight of tray is taken and water content is determined as:

Calcium Carbide Method

This method is based on the fact that, when water reacts with calcium carbide, if forms acetylene gas. The reaction is as follows:

CaC₂ + H₂O = C₂H₂ + H₂O

The water content of the soil is determined indirectly by measuring the pressure of the acetylene gas formed. The instrument used is called “moisture tester”.

Radiation Method

In this method, radioactive isotopes are used. Generally, Cobalt-60 is used which is placed in the form of the capsule in a device. The device is lowered into the ground with the steel casing. The steel casing has an opening from which rays come out and received at the other end where a detector is placed in another steel casing inside the ground.

The radioactive material emits neutron which strikes with the hydrogen atom present in the water molecules and loses energy. The loss of energy is proportional to the quantity of water present in the soil. This method is extremely helpful in determing the water content of the soil in in-situ conditions.

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